The Edison Module Hardware Guide on page 7 says that the internal memory of the Edison is 4GB eMMC, and as far as I understand it is used to store the image so you should be able to use it as you please. You can check this blog, it's about the partition setup, and you might find it helpful. Also another suggestion is that you can type df -h in the Linux shell to see the current size of your Edison partitions to have them as a reference.